The disclosures herein relate generally to heart valves and more particularly to tri-leaflet polymer valves.
Heart valves include mechanical valves, tissue valves and polymer valves. A heart valve is implanted into an annular opening in a heart created when a diseased valve is removed. The valve can be secured in the annulus of the opening through the use of sutures or pins that penetrate the host tissue and an outside edge of the valve. Alternatively the valve can then be secured in the annulus by suturing the host tissue to the sewing ring. Heart valves function essentially as a one-way check valve.
Early heart valve prostheses included ball-and-cage valves and disc-and-cage valves in which a ball or a disc was housed in a cage. One side of the cage provided an orifice through which blood flowed either into or out of the heart, depending on the valve being replaced. When blood flowed in a forward direction, the energy of the blood flow forced the ball or disc to the back of the cage allowing blood to flow through the valve. When blood attempted to flow in a reverse direction, or "regurgitate", the energy of the blood flow forced the ball or disc into the orifice in the valve and blocked the flow of blood.
A bi-leaflet valve comprised an annular valve body in which two opposed leaflet occluders were pivotally mounted. The occluders were typically substantially rigid, although some designs incorporated flexible leaflets, and moved between a closed position, in which the two leaflets were mated and blocked blood flow in the reverse direction, and an open position, in which the occluders were pivoted away from each other and did not block blood flow in the forward direction. The energy of blood flow caused the occluders to move between their open and closed positions.
A tri-leaflet valve comprised an annular valve body in which three flexible leaflets were mounted to a portion of the valve body, called a "stent," located at the circumference of the annulus. Some tri-leaflet valves used rigid leaflets. When blood flowed in the forward direction, the energy of the blood flow deflected the three leaflets away from the center of the annulus and allowed blood to flow through. When blood flowed in the reverse direction, the three leaflets engaged each other in a coaptive region, occluded the valve body annulus and prevented the flow of blood. The valve leaflets were made from tissue, such as specially treated porcine or bovine pericardial tissue or from a man-made material such as polyurethane or another biocompatible polymer.
In one specific example, U.S. Pat. No. 4,265,694 discloses an artificial tri-leaflet heart valve wherein the entire free margin includes a thickened lip portion combined with reinforcing lines radiating from the base of the valve for reinforcing the leaflets.
In U.S. Pat. No. 4,364,127, a prosthetic heart valve constructed of hemo-compatible materials that is anatomically and functionally similar to the natural aortic valve is disclosed. The heart valve is a tri-leaflet type which has its formed leaflets heat set in a partially open position to reduce pressure required to open the leaflets in response to blood flowing therethrough.
U.S. Pat. No. 4,473,423 discloses an artificial heart valve having thin, seamless leaflets which converge to the center of a frame from the frame's inner wall. The leaflets each have a convex outflow surface and a concave inflow surface. The leaflets meet along adjacent edges to form cusps. Sinus valsalvae sections of the valve are formed as rounded recesses defined in the valve frame's inner wall as continuous curved profiles of the respective leaflet concave surface. The valve is fabricated by vacuum molding techniques whereby layers of elastomer are vacuum formed to comprise the leaflet and sinus valsalvae portions. The leaflets are all formed from two or more layers of elastomer which are cut to define the leaflet edges or commissures. One elastomer layer extends along the frame recess to provide continuity for each leaflet and its sinus valsalvae. The resulting structure has no rims or seams in the inflow or outflow paths.
In U.S. Pat. No. 4,778,461, a heart valve prosthesis for replacing the aortic valve or the pulmonary valve, comprises a support ring with at least two commissure supports and flexible cusps, is characterized in that the height of the support ring including the commissure supports is less than the total height of the heart valve prosthesis.
In U.S. Pat. No. 4,888,009, a prosthetic heart valve comprises a suture ring supporting a stent which surrounds a conduit bearing a plurality of flexible valve leaflets. The conduit extends beyond the end of the suture ring.
U.S. Pat. No. 5,116,564 discloses a method of producing flexible closing members, especially artificial heart valves. The housing of the closing member is radially expanded, the closing element of the closing member is formed as a substantially plane two-dimensional element, and the plane two-dimensional element is connected to the housing in the expanded condition of the same. This method is preferably realized as a dip method according to which the closing element is shaped and formed to the housing in a single working step. The described flexible closing member is an artificial three-sail heart valve which is characterized by a special shape of the three closing elements.
In U.S. Pat. No. 5,500,016, a flexible leaflet heart valve, to replace natural aortic or pulmonary valves of the heart, includes a frame and flexible leaflets attached to the frame. Each flexible leaflet forms part of a surface of revolution having its axis of revolution substantially orthogonal to the direction of blood flow through the valve.
U.S. Pat. No. 5,562,729 discloses a multi-leaflet heart valve composed of a biocompatible polymer which, simultaneously imitates the structure and dynamics of biological heart valves and avoids promotion of calcification. The valve includes a plurality of flexible leaflets dip cast on a mandrel, which leaflets are then bonded with a bonding agent to the interior surfaces of a plurality of struts on a metal-reinforced prosthetic stent. The leaflets open and close in response to the pumping action of the heart. The leaflets and the polymer components of the prosthetic stent are manufactured of biocompatible polymers exhibiting intrinsic calcification-resistant properties.
An important consideration in prosthetic heart valve design is the durability of the heart valve. Replacing a prosthetic heart valve after it has been implanted is inconvenient and expensive for the patient. Mechanical valves may enhance the possibility of clotting. Therefore, patients using mechanical valves are typically required to take anti-coagulation medication. Also, mechanical valves are noisy which is most disconcerting to patients. Patients having tissue valve implants are usually not required to take anti-coagulation medication. Tissue valves are not noisy like mechanical valves however, they are also not very durable.
Polymer valves are typically molded to a desired shape and then cut to form the free margins of the leaflets. One source of prosthetic heart valve failure is tearing. Cutting to form the free margins of the leaflets may introduce imperfections in the polymer which can lead to eventual failure of the valve. Therefore, cutting creates durability problems at the cut surfaces.
A factor in heart valve design is the consideration of energy loss. It is important to design a heart valve in a manner that will provide low energy loss. Energy loss considerations include forward pressure drop, leakage volume and closing volume. A major limitation of many known polymer valves is that, due to their geometry, they have significant planar coaption surfaces in the at rest or natural-state condition to provide good closure characteristics, but this causes the heart to work harder to open the valve. Other known polymer valves include geometries that are easier to open but the neutral position gap area is exaggerated. This increases closing volume which increases energy loss.
Finite element analysis of a flexible heart valve leaflet predict that the leaflet can buckle along the free margin during valve closing. This is because elements of the valve are compressed inwardly during valve closing. For example, the commissures are moved radially inwardly during valve closure. In addition, the free margins coapt, or engage during valve closure. As a result, the leaflets are exposed to compressive loading and become unstable. Buckling of the thin, flexible leaflets may occur and is most pronounced at the free margin. As the free margin buckles, a portion of the leaflet, between the mid-point and the commissure, bends away from a coaption plane formed by coapting free margins of an adjacent leaflet. Buckling is a complex structural failure and is difficult to analyze. If buckling at the free margin does occur, it can prevent complete coaption of the free margins. In addition, it can cause very high local stresses with each heart beat which can fatigue the valve.
Therefore, what is needed is a molded heart valve preferably of polymer or other suitable material, which includes thin, flexible leaflets that open easily under low opening pressure, that maintains stability under compressive loading that can occur on the free margin of the heart valve leaflet during closing, and which can maintain the free margin in tension during closing to avoid the problems associated with compressing the free margin as set forth above.